1. Field of the Invention
The present invention relates to a fixing device having an excessive temperature rise prevention member and an image forming apparatus including the fixing device.
2. Discussion of the Background Art
In the past, it is well known that temperature of a heating member included in a fixing device of an image forming apparatus, such as a copier, a printer, etc., sometimes excessively increases and damages the fixing device and/or creates an abnormal image when a printing operation is completed as discussed in the Japanese Patent Application Laid Open Nos. 2004-102104 and 2000-75707.
Specifically, the fixing device includes a heating roller heated by either a heater or an electromagnetic induction heating system and a fixing roller serving as a heating member. In such a device, heat is absorbed by a printing medium via a fixing member, such as a fixing belt, a fixing roller, etc., during a printing operation, and temperature of a heating member does not abnormally increases. However, since heat stored in a core metal or the like included in the heating member suddenly stops traveling to the printing medium right after completion of the print operation, temperature of the heating member abnormally increases and causes so called overshoot. This tends to happen immediately when a consecutive printing operation is terminated. Further, the higher the printing speed (i.e., cpm) or the larger the basic weight of the printing medium conveyed, the more problem occurs.
To prevent such overshoot, various attempts have been presented. For example, the Japanese Patent Application Laid Open No. 2004-102104 discusses that a heating operation for heating printing mediums in a print job is stopped before printing of the last page thereof is completed. The Japanese Patent Application Laid Open No. 2000-75707 discusses that a cooling fan cools a fixing roller when a temperature detection device arranged on a fixing roller detects prescribed temperature. The Japanese Patent Application Laid Open No. 2006-227374 discusses that air is blown and cools a surface of a heating roller at the end of printing. The Japanese Patent Application Laid Open No. 2006-119430 discusses that temperature of a region of a heating member, where a printing medium does not pass through, is detected after the end of consecutive printing, and a cooling fan blows cooling air to the heating member when more than a prescribed temperature is detected.
In the above-mentioned various conventional arts, the overshoot can be effectively suppressed as far as the cooling fan normally operates. However, when the cooling fan goes wrong and impossible to provide the cooling air to the heating member, temperature of the heating member increases and overshoot cannot be suppressed. To resolve such a problem, an excessive temperature rise prevention device including a thermostat (TM) is provided in the vicinity of the heating member to forcibly cut off power distribution to the heating member when temperature of the heating member excessively increases as discussed in the Japanese Patent Application Laid Open No. 2006-119430.
So as to appropriately operate an excessive temperature rise prevention device, a cooling fan is provided to blow cooling air to the excessive temperature rise prevention device during a normal printing operation so that the excessive temperature rise prevention device does not go wrong as discussed in the Japanese Patent Application Laid Open No. 2006-172781. When a temperature detection device provided in the vicinity of the heating member detects a prescribed level, the cooling fan stops operation, and the excessive temperature rise prevention device starts operation and cuts off the power distribution to the heating member.
The cooling fan of the Japanese Patent Application Laid No. 2006-172781 operates when a printing medium is conveyed and receives printing, but stops the operation when the printing medium is stopped conveying and receiving the same. Since heat is not absorbed from the heating member to the printing medium when power distribution is stopped as the printing operation stops, temperature of the heating member and the excessive temperature rise prevention device increase. As a result, the excessive temperature rise prevention device is forcibly turned on. The cut off condition of the power distribution is sometimes maintained even when the power supply to the heating member is turned off, and it is impossible to restart power supplying even it is repeatedly attempted. As a result, repair is needed to initialize a system.
Now, an erroneous operation of the excessive temperature rise prevention device is described with reference to FIG. 10. As shown, a conventional fixing device 1 includes a halogen lamp 2 serving as a heating member, a temperature detection device 3, such as a thermister, etc., an excessive temperature rise prevention device 4 having a thermostat (TM), a cooling device 5 having an air blower fan, and a control device 6. When a heating roller 7 serving as a heated member is normally heated, i.e., when a printing medium is conveyed and receives printing, the control device 6 controls the cooling device 5 to cool down and maintains the excessive temperature rise prevention device in a low temperature. When the temperature detection device 3 detects a prescribed surface temperature of the heating roller 7, the control device 6 turns off and stops the cooling device 5 to cool the excessive temperature rise prevention device 4. As a result, temperature of the excessive temperature rise prevention device 4 sharply increases, and forcibly cuts off power distribution to the halogen lamp 2. Accordingly, the heating roller 7 is prevented from abnormally increasing its temperature (i.e., overshoot). In these days, a color copier is expected to speed. For example, a conventional color copier having a performance of 40 cpm increases its temperature up to about 230 degree centigrade when causing overshoot as indicated by a curvature 1 in FIG. 11. Whereas a recent color copier having a performance of more than 75 cpm increases its temperature to about 270 degree centigrade when causing the same as indicated by a curvature 2. Specifically, as the color copier is speeded, temperature at the overshoot increases as understood from the drawing.
Further, in view of energy saving of recent tendency, a warm up time needed for a fixing section is expected to be short. Thus, a heating roller 7 is increasingly thinned and employs a heater capable of starting up at high speed. However, such speeding by thinning extraordinarily narrows applicability of the excessive temperature rise prevention device 4 having the thermostat or the like. Specifically, to ensure the prevention of excessive temperature rise of the fixing device 1 owing to overdrive at the time of start up, a thermostat operable at low temperature is needed. That is, a temperature rise speed of a bimetal section of the thermostat is slower in comparison with that of the heating roller. When the low temperature operation performance thermostat is used as an excessive temperature rise prevention device and a printing operation of the fixing device is completed and a printing medium is not conveyed, temperature of the heating roller 7 increases, and the thermostat unexpectedly starts an operation. Then, the thermostat forcibly highly provably cuts off power distribution to the halogen lamp 2, erroneously. Such a forcible cutting off operation is hardly initialized and needs repair by an expert person. Such a phenomenon tends to occur either when a printing medium is jammed on a printing medium conveyance path of the image forming apparatus or when electric service stops.
Temperature changes in the heating roller 7 having the thermostat as a temperature rise prevention device are now described with reference to curvatures of FIG. 12. A shown, the thermostat starts an operation at temperature X, such as 190 degree centigrade. As understood therefrom, the thermostat possibly erroneously operates after the fixing device 1 stops printing in the above-mentioned conventional fixing device.
Specifically, since the cooling device 5 stops when the heating roller 7 stops feeding a sheet, temperature of the thermostat increases as shown by the curvature 4 as that of the heating roller 7 increases as indicated by the curvature 3. Then, the thermostat reaches the operation temperature X (e.g. X1) and is activated, thereby forcibly cutting off power distribution to the halogen lamp 2. As a result, the power distribution to the halogen lamp 2 is stopped due to stop of the printing operation. However, the forcible cutting off condition is maintained and cannot be initialized even if it is attempted to supply power and start printing. To initialize and recover the power distribution to the halogen lamp, the private repair specialist should be called.